The present invention relates to scale reading apparatus.
A known form of opto-electronic scale reading apparatus for measuring relative displacement of two members comprises a scale on one of the members, having scale marks defining a periodic pattern, a read head provided on the other member, means for illuminating the scale, periodic diffraction means for interacting with light from the scale marks to produce interference fringes having movement relative to the read head and detecting means in the read head responsive to the fringes to produce a measure of the displacement.
An example of such apparatus is disclosed in EP-A-0 207 121 and also U.S. Pat. No. 4,974,962, each of which shows the means for illuminating and the periodic diffraction means in the read head. U.S. Pat. No. 4,926,566 discloses a method of producing a scale, in the form of a flexible tape produced by rolling, the pitch of the scale marks being 20 xcexcm or 40 xcexcm for example. The illuminating means, the diffraction means and the detecting means responsive to the fringes may be integrated in the read head in the manner described in U.S. Pat. No. 5,302,820.
The above forms of apparatus basically measure linear displacement. However, for measuring rotary displacement, such a scale may be held around a cylindrical surface which rotates in use with a shaft or other rotary part relative to a read headxe2x80x94see DE-A-197 51 019 for example. Such apparatus is typically called a rotary or angle or shaft encoder for example.
Referring to FIG. 1, there is shown schematically a loop of scale 1 having scale marks defining a periodic pattern, the loop of scale being fixed for example by adhesive to the outer circumferential surface of a rotary ring (not shown in FIG. 1), reference numeral 2 designating a read head in which are means for illuminating the scale marks, periodic diffraction means for interacting with light from the scale marks to produce interference fringes having movement relative to the read head and detecting means in the read head responsive to the fringes to produce a measure of the rotary displacement of the shaft or other rotary part to which the ring is attached in use.
However, a problem is that the rotary ring might not sit concentrically on the rotary element on which it is mounted.
According to the present invention there is provided a rotary ring for use in scale reading apparatus, the ring carrying on an outer circumferential surface a length of scale having scale marks defining a periodic pattern and readable by a read head of such apparatus, wherein an inner circumferential surface of the ring is provided by a tapered surface at an angle relative to the axis of the ring.
The length of scale could be for use in opto-electronic scale reading apparatus in which diffraction means interferes with light from the scale marks or the scale reading apparatus could be of the magnetic or capacitive type for example.
The present invention also comprises scale reading apparatus comprising a rotary ring according to the present invention and a read head for reading the marks on the length of scale.
The angle that the tapered surface makes with the axis of the ring could be 15xc2x0, for example.